The invention relates to a method and an apparatus for the thermal bonding of a base part of a packaging with a cover film, and to a method and an apparatus for packaging contact lenses in accordance with the preamble of the respective independent patent claims.
Methods and apparatuses of the above-mentioned kind are already used today in a large number of variants where a specific article, especially a contact lens, is to be packed in a tightly sealed manner in a packaging. This is generally accomplished by introducing a contact lens into a depression of a base part of a packaging, dispensing a predetermined amount of a preserving solution into the depression, then placing onto the packaging a cover film that can be thermally bonded to the base part of the packaging, and finally thermally bonding the cover film to the packaging. That operation may then be followed by sterilisation. The packaging containing the contact lens can then be delivered to the customer or to the optician or ophthalmologist.
It will be readily apparent that especially in the field of the packaging of contact lenses, but also, for example, in the fields of foodstuffs, pharmaceuticals, medicinal articles and some other fields, certain demands are made of the packaging. Those demands may relate especially to the sealing (against leakage or against the ingress of foreign substances) of the packaging, but they may also relate to the adhesion of the cover film to the base part of the packaging, to the impermeability of the packaging to certain substances, or to the force required to remove the film (peeling force). If a packaging is to be able to meet those demands, the parameters during the thermal bonding of base part and cover film (duration, pressure, temperature) must always remain within predetermined limits.
In packaging technology there are various techniques for joining two materials to one another. These include inter alia adhesive bonding with hot or cold glue, welding and xe2x80x9csealingxe2x80x9d. Whereas in adhesive bonding the join is generally only in the form of a point or line, welding or sealing are suitable especially also for joins made over a larger area. The difference between welding and sealing lies in the nature and the characteristics of the materials to be joined. In packaging technology, xe2x80x9cweldingxe2x80x9d is referred to when two identical materials are joined together under the action of pressure and temperature (without the involvement of any other materials). In the case of xe2x80x9csealingxe2x80x9d, however, at least one of the partner materials is provided with a sealable coating. In both methods, therefore, the partner materials are joined together under the action of pressure and temperature, but in the case of sealing the materials to be joined to one another are generally different. Sealing is used most commonly in the field of blister packagings, in which primarily clear, deep-drawable plastics films are sealed to a cardboard card, the cardboard generally having been coated with a sealable coating of sealing varnish or, for example, with a polyethylene film.
In the packaging of contact lenses it is customary for a base part, which can be produced, for example, by injection-moulding, to be bonded to a film that has been covered with a sealable coating. The bonding of base part and cover film is effected after the lens has been introduced into a suitable depression in the base part and after a preserving solution, for example saline, has been dispensed into the depression. Such a packaging is described, for example, in EP-A-0 680 895. That packaging is said to be sealed against the ingress of foreign substances from the outside and is said to prevent the saline from leaking out. The adhesion of the cover film to the packaging must be such that the packaging is not damaged during sterilisation, which normally takes place in an autoclave. In addition, the peeling force required to remove the film from the base part, that is to say to open the packaging, should not exceed a maximum value in order that the user is able to open the packaging with a reasonable amount of effort.
Known apparatuses for the thermal bonding of a base part to a cover film, as used, for example, in the sealing of blister packagings, are constructed essentially as follows: a holding device or cradle is provided with a number of depressions for accommodating the base parts. The holding device is connected to an adjusting device which can be moved towards a bonding unit, for example with the aid of a pneumatic drive means. The bonding unit comprises a bulky metal heating plate having a large heat capacity and it too is arranged to be moved towards the holding device or the cradle with the aid of a further adjusting unit which may include, for example, a pneumatic drive means. In the heating plate of the bonding unit there are arranged a number of heating elements which heat the heating plate to a predetermined temperature. Below the heating plate, that is to say facing the holding device or the cradle, a sealing plate is releasably connected to the heating plate. The sealing plate is provided with a number of contact elements, so-called xe2x80x9cdiesxe2x80x9d. The end face of the dies constitutes the actual pressure face that comes into contact with the cover film during sealing. That pressure face determines substantially also the shape of the join along which the cover film is sealed to the base part. Temperature sensors are provided in the heating plate and the sealing plate. Those temperature sensors are connected to a regulating means which is used to control the length of time for which the heating elements are switched on.
That known apparatus operates as follows: the base parts are held ready in the depressions of the holding device (cradle) and the cover film is placed onto those base parts. Using the adjusting units to move the holding device (cradle) and the bonding unit, the latter are moved towards one another until the pressure faces of the dies press the cover film against the base parts at a predetermined pressure. As the film is pressed against the base part by means of the dies, the sealable coating is heated and, under the action of pressure and temperature, base part and cover film are bonded together.
That known apparatus is basically perfectly efficient, but it does still have disadvantages especially in respect of the above-mentioned demands that must be met when packaging contact lenses. The movement of the holding device and/or the bonding unit must be such that those surfaces of the holding device and of the bonding unit which transfer the sealing pressure to the packaging are very exactly parallel to one another. Even very slight fluctuations can result in significant variations in the sealing pressure and therefore in a seal that does not meet the demands mentioned.
Since the sealing pressure is applied directly by way of the adjusting units, the adjusting unit in question, or its drive means, must operate very accurately. According to the nature of the adjusting unit (mechanical, hydraulic or, as mentioned above, pneumatic), the control means required for that purpose may be very expensive.
The heating elements in the heating plate and the pressure face (that is to say the end face of the dies) are a relatively large distance apart. As a result of the large heat capacity (inertia) of the bulky heating plate and as a result of the heat capacity (inertia) of the sealing plate, a relatively long time elapses before a change in temperature occurs at the pressure face. A rapid change in temperature at the point that is crucial for the quality of the seal, namely at the pressure face, is therefore not possible. In addition, the temperature at the pressure face also continues to rise for a considerable period when the heating elements are switched off again after a heating operation.
Even with the aid of the temperature sensors it is very difficult to achieve accurate regulation of the temperature at the pressure face (end face of the die). If the heat supply is regulated solely on the basis of the temperature sensor that is arranged in the sealing plate as close as possible to the dies, the heating plate/sealing plate system has a tendency to xe2x80x9covershootxe2x80x9d because it is has a high inertia. Therefore if, for example, the temperature sensor signals that the temperature at the site of the temperature sensor (i.e. close to the die) is too low, the heating elements receive the signal to heat. Since, however, the heating plate/sealing plate system has a high inertia, the temperature at the site of the temperature sensor does not rise immediately. As a consequence, the temperature sensor signals that further heating is required, with the result that the heating plate is heated further, and even when, after a while, the temperature at the site of the temperature sensor has reached the desired temperature, the heat previously supplied to the heating plate will result in the temperature of the heating plate being transmitted to the sealing plate and therefore to the dies and the pressure faces thereof, resulting in an excessive increase in temperature (overshoot). If, however, the heat supply is regulated solely on the basis of the temperature sensor arranged in the heating plate, then although the heating elements can be regulated relatively accurately (and thus thermal damage to the heating elements (burning out) can be avoided), the actual temperature at the pressure face will be somewhat lower than the temperature at the heating elements. If the heat supply is regulated with the aid of both temperature sensors, the heating plate/sealing plate system will still have a tendency to overshoot as a result of its inertia. If the transient behaviour is recognised and taken into account when supplying heat, i.e. if an attempt is made to avoid overshooting, a rapid change in temperature at the crucial site, namely at the pressure face (end face of the dies), will again not be possible because, as a result of the large heat capacity (inertia), the supply of heat must take place correspondingly more slowly. If a heating element fails, this will not necessarily be noticed because the other heating elements will assume the function of the defective heating element. However, the temperature distribution at the different pressure faces (end faces of the dies) will consequently be very much less accurate, resulting in seals that do not meet the above-mentioned demands. This cannot be tolerated, however, particularly in the case of fully automated production and packaging of large numbers of contact lenses.
It is therefore an aim of the invention to propose a method and an apparatus for the thermal bonding of a base part and a cover film of a packaging, especially a method for the welding or sealing of a base part and a cover film of a packaging for contact lenses, that does not have the mentioned disadvantages. A further aim is to propose a method of packaging contact lenses with which it is possible to obtain packagings that meet the mentioned demands even in the case of fully automated production and packaging of large numbers of contact lenses.
In the thermal bonding method according to the invention, therefore, for the purpose of generating the mechanical pressure and causing heat to act on the join there is used a contact plate of low heat capacity which, during the thermal bonding, is pressed against the surface of the cover film remote from the contact surface. Furthermore, the temperature of the contact plate is measured in the immediate vicinity of the cover film. In the event of the measured temperature deviating from a predetermined tolerance range around a desired temperature, the temperature of the contact plate is within a very short time adjusted so that it again lies within the predetermined tolerance range around the desired temperature. That is possible only as a result of the low heat capacity of the contact plate which, accordingly, has only a low degree of inertia. In other words: when the temperature measured in the vicinity of the cover film lies outside the tolerance range around the desired temperature, and accordingly, for example, further heating is carried out, the temperature in the vicinity of the cover film is very rapidly returned to the tolerance range. Large-scale overshooting is likewise avoided.
Advantageous variants of the method according to the invention will be found in the dependent patent claims. They relate, for example, to the feature that during the thermal bonding a predetermined, substantially uniform mechanical pressure is generated along the entire join. This can be achieved, for example, by using a pivotally mounted die to generate the substantially uniform mechanical pressure along the join, the contact plate being arranged on the end of the die that faces the base part of the packaging. For that purpose it is possible to use a die that is mounted so as to be movable against the force of a spring towards a frame arranged in a fixed spatial position.
Further advantageous variants of the method relate to the following features: a number of base parts are held ready simultaneously in a holding device; a number of dies pivotally mounted on a common fixed frame are used; and a number of base parts are thermally bonded to a cover film simultaneously by moving the holding device towards the dies or towards the frame on which the dies are mounted, so that as a result of the transmission of heat and pressure to the contact surfaces of base part and cover film the latter are thermally bonded to one another along the join.
The apparatus according to the invention is distinguished by the fact that the die has a heatable contact plate of low heat capacity which is arranged on the end of the die facing the holding device and, during the thermal bonding, presses against the surface of the cover film remote from the contact surface along the join. On the contact plate there is arranged, in the immediate vicinity of the pressure face which presses against the cover film, a temperature sensor which measures the temperature of the contact plate and which, in the event of the temperature deviating from a predetermined tolerance range around the desired temperature, immediately generates a corresponding signal and passes it on to a fast regulating means which immediately regulates the temperature of the contact plate so that it again lies within the predetermined tolerance range. The advantages correspond to the advantages already mentioned in connection with the method according to the invention.
Advantageous embodiments will be apparent from the features of the dependent claims. They relate, for example, to the fact that the die may be pivotally mounted. Furthermore, the die may have on the face of the contact plate remote from the holding device a heating element, which heats the contact plate, and an insulating body in order that substantially all the heat generated by the heating element is transmitted to the contact plate and not to the environment. The heating element can be constructed in the form of a thin ceramics plate on which there are provided on the one hand a resistance heating device having two metal connection pads and on the other hand resistive lines that connect those two connection pads to one another in the manner of a conductor. That heating arrangement is especially simple structurally and is also reliable. The power supply for supplying the resistance heating device with electrical voltage comprises in one embodiment two resilient metal pins which extend through the insulating body and the ends of which are in contact with the connection pads of the resistance heating device.
Further embodiments relate to the fact that the die is mounted so as to be movable against the force of a spring towards a frame arranged in a fixed spatial position. Further embodiments are distinguished by the provision of a number of dies that are mounted on a common fixed frame. Furthermore, the holding device has a number of depressions for accommodating base parts of a packaging, and the holding device, together with all the base parts, can be moved by means of an adjusting unit towards the dies and towards the frame on which the dies are mounted.
The method for packaging contact lenses according to the invention is distinguished by the fact that, for the thermal bonding of base part and cover film, use is made of one of the above-mentioned methods for the thermal bonding of base part and cover film. In an advantageous variant, after the base part and cover film have been thermally bonded, the packaging so produced is sterilised. Preferably the packaging is conveyed to an autoclave for sterilisation. Prior to sterilisation, a number of packagings can first be conveyed to a magazine and then the packagings contained in the magazine can be sterilised together.
The contact lens packaging apparatus according to the invention is distinguished by the fact that the device used for the thermal bonding of base part and cover film is one of the above-mentioned embodiments of apparatuses for the thermal bonding of base part and cover film. In one embodiment, the apparatus includes a device for sterilising the packaging after the base part and cover film have been bonded. That device for sterilising the packaging may be, for example, an autoclave. A magazine may be provided, to which a number of packagings are conveyed prior to sterilisation, and the magazine is then introduced into the autoclave for the purpose of sterilising the packagings.